1. Field of the Invention
The present invention relates to fast reactors having a reflector control system and neutron reflectors therefor, and more particularly, relates to a fast reactor having a reflector control system and a neutron reflector therefor, the fast reactor controlling reactivity of a reactor core through adjustment of leakage of neutrons leaked therefrom by moving the neutron reflector in a vertical direction which is disposed outside the reactor core immersed in a liquid metal coolant.
2. Description of the Related Art
In general, in nuclear reactors such as a fast reactor, as a method for controlling the reactivity of a reactor core, for example, a method using a control rod and a method for controlling a leak rate of neutrons may be mentioned as disclosed in Japanese Patent Nos. 2835161 and 2892824.
Of the methods mentioned above, a method for controlling a leak rate of neutrons is suitably used for a nuclear reactor having a small reactor core, and by way of example, structures shown in FIGS. 33, 34, and 35 may be mentioned. For example, in a nuclear reactor 200, as shown in FIG. 33, a reactor core 202 is placed at a central position of a reactor vessel 201 and is enclosed by a core barrel 203. In addition, the reactor vessel 201 is filled with a liquid metal coolant 204 such as sodium metal.
FIG. 34 is a schematic cross-sectional view of the fast reactor shown in FIG. 33 taken along the line indicated by arrows XXXIV, the fast reactor being viewed along the direction shown by the arrows XXXIV.
For example, as shown in FIG. 34, in an area enclosed by the core barrel 203, 18 hexagonal fuel assemblies 205 are disposed, and at the central place of the fuel assemblies 205, a channel 206 for a neutron absorbing rod (hereinafter referred to as “neutron absorbing channel”) is provided. The neutron absorbing channel 206 is used for the reactivity control of the reactor core 202 and is pulled out upward in operation. In addition, reference numeral 207 indicates a guard vessel, that is, a protective container surrounding the reactor vessel 201.
In FIG. 33, outside the core barrel 203, a partition 208 is provided with a space interposed therebetween, and in addition, in this space interposed between the partition 208 and the core barrel 203, a neutron reflector 209 which moves along the reactor core 202 and a neutron reflector drive device 210 are provided. The neutron reflector drive device 210 moves the neutron reflector 209 by driving a drive rod 211 fitted thereto.
In addition, the space between the core barrel 203 and the partition 208 is a movement zone Z in which the neutron reflector 209 is moved during operation of the reactor core 202 and through which the coolant 204 is allowed to flow. Furthermore, between the partition 208 and the reactor vessel 201, a great number of neutron shielding members 212 are provided. The neutron shielding members 212 are provided for restricting the amount of neutron radiation of the reactor vessel 201 to a predetermined value or less which is determined for each plant in accordance with the life thereof, and are each formed of a plurality of neutron shielding rods 212a. 
As the neutron shielding member 212, in addition to a structural member formed of stainless steel, for example, there may be mentioned a pin accommodating a B4C ceramic containing boron which has a high neutron absorbing ability, a metal, such as hafnium or tantalum, or a material containing a compound of the material mentioned above.
In addition, in Japanese Patent No. 3126502, a technique of enhancing a reactivity control ability of the neutron reflector 209 has been disclosed in which, for example, as shown in FIG. 35, a neutron absorber or a neutron transmitting material (hereinafter referred to as “cavity”) 214, which has a lower neutron reflection ability than that of the coolant 204, is placed at an upper region of a neutron reflector.
The reactor core 202, core barrel 203, partition 208, neutron reflector 209, and neutron shielding members 212 are provided on a supporting structure 215 formed at the bottom portion side of the reactor vessel 201. The supporting structure 215 has a great number of holes through which the coolant is allowed to flow, and in addition, between the supporting structure 215 and the reactor vessel 201, a bottom plenum 216 is provided.
In addition, at a head portion side of the neutron shielding members 212 provided between the partition 208 and the reactor vessel 201, an intermediate heat exchanger 218 and an electromagnetic pump 219 are provided. The intermediate heat exchanger 218 is provided with a secondary coolant-path pipe 220.
An opening of a head portion of the reactor vessel 201 is closed with a shielding plug 222, and the inside of the reactor vessel 201 is filled with the liquid metal coolant 204 such as liquid sodium. In addition, between the coolant 204 and the shielding plug 222, a head plenum 223 filled with an inert gas is formed.
FIG. 36 is a vertical cross-sectional view of the fuel assembly 205 of the nuclear reactor and the vicinity of the fuel assembly 205.
As shown in FIG. 36, the fuel assembly 205 has a fuel pin 226 received in a hexagonal-shaped wrapper tube 225 made of stainless steel and neutron shielding members 227a and 227b provided at a head portion and a bottom portion, respectively, of the wrapper tube 225.
The fuel pin 226 is formed of a fuel portion 226a and a plenum portion 226b in which gas components generated by nuclear fission are to be enclosed. This fuel pin 226 helps the coolant 204 to flow through the wrapper tube 225 by a wire wrap or a grid design (both of which are not shown in the figure), and one end of the fuel pin 226 is inserted into the wrapper tube 225.
In addition, the fuel assembly 205 has a coolant outlet 229 at a head portion side and a coolant inlet 230 at a bottom portion side, and in addition, an entrance nozzle 231 having the coolant inlet 230 therein is fitted to a core holding plate 232.
In a nuclear reactor having the structure described above, the coolant 204 is circulated by a drive force of the electromagnetic pump 219 in the reactor vessel 201 in a direction indicated by arrows and is allowed to flow inside the reactor core 202. The coolant 204 which flows inside the reactor core 202 takes away heat therefrom.
The coolant 204, which flows inside the partition 208 from the bottom portion side to the head portion side and which takes off the heat from the reactor core 202, enters the reactor core 202 and absorbs the heat generated by nuclear fission therein, and hence the temperature of the coolant 204 is increased. The coolant 204 thus heated enters the intermediate heat exchanger 218 to exchange heat with a secondary coolant, and as a result, the temperature of the coolant 204 is decreased.
After the coolant 204 thus cooled flows through the intermediate heat exchanger 218, the pressure of the coolant 204 is increases by the drive force of the electromagnetic pump 219 and is supplied to the supporting structure 215 through the area outside the partition 208 in which the neutron shielding members 212 are provided. Furthermore, after supplied to the bottom plenum 216, the coolant 204 flows to the bottom portion side of the reactor core 202 and again enters the reactor core 202, so that the coolant 204 is repeatedly circulated as described above.
On the other hand, by the drive force of the neutron reflector drive device 210 provided on the shielding plug 222, the neutron reflector 209 is moved in the space between the core barrel 203 and the partition 208 along the reactor core 202 so as to adjust the leakage of neutrons from the reactor core 202 and so as to compensate for the change in reactivity caused by the burn-up in the reactor core 202. As shown in FIG. 37, the reactor core 202 is not divided and is formed of a fuel containing a fissile material at a constant ratio in the axial direction.
Next, the analysis of a fast reactor core having specifications shown in FIG. 38 was performed by way of example. That is, the following are assumed that a fast reactor core, which has a thermal power of approximately 130 MW, a core diameter of approximately 130 cm, and a core height of 200 cm and which uses a metal compound U—Zr of a uranium concentrate as a fuel, is operated for approximately 30 years without refueling, and that, in order to compensate for the change in reactivity caused by the fuel burn-up, the operation is performed while a stainless steel-made reflector having a length of 200 cm and a thickness of 30 cm is being lifted up at a predetermined rate. The calculated results of the change in reactivity of the fast reactor in this case are shown in FIG. 39.
In the case described above, the following are also assumed that when the top end of the reflector is placed at a position lower than the bottom of the reactor core, the reactor is in a subcritical state, that is, in a shutdown state; at the initial burn-up stage, when the reflector is lifted up so as to cover the reactor core in a region from the bottom to a position of approximately 75 cm therefrom in an upward direction, the critical state is obtained; and subsequently, while the operation is being performed at a constant power, the decrease in reactivity caused by burn-up is compensated for by the rise of the reflector, and the entire reactor core is covered with the reflector after 30 years. According to the results thus obtained, during operation for up to approximately 15 years, which is at the middle burn-up stage, the reactivity is maintained constant or is slightly increased. However, after the middle burn-up stage, the reactivity is gradually decreased and becomes considerably low at the last burn-up stage. In order to find out the reasons for this tendency, the reactivity is divided into two components as shown in FIG. 39. That is, one component is the change in reactivity caused by the change in composition of the fuel resulting from the burn-up thereof, and the other component is the change in reactivity caused by the movement of the reflector in the axial direction.
As shown in FIG. 39, due to the change in composition of fuel caused by the burn-up, the reactivity tends to be decreased at a constant rate. The amount of a fissile material, U-235, contained in fuel elements is monotonously decreased by burn-up. However, since a fissile material, Pu-239, is produced from U-238, the decrease in reactivity is suppressed at the initial burn-up stage. Although, when a certain amount of Pu-239 is produced as the burn-up proceeds, by the burn-up of Pu-239 itself and by the decrease of U-238 which is a parent nuclide of Pu-239, the production amount of Pu-239 tends to be saturated. Hence, the reactivity tends to be gradually decreased.
On the other hand, by the movement of the reflector in the axial direction, the reactivity is increased at an approximately constant rate at the initial burn-up stage. However, after the middle burn-up stage, the rate of increase in reactivity tends to be gradually decreased. As a result, in combination of the burn-up effect of fuel and the movement effect of reflector, as shown in FIG. 39, the total reactivity is slightly increased until the middle burn-up stage, and subsequently, the reactivity is decreased with time.
In order to increase the reactivity at the last stage, when the reflection ability of the reflector is increased (for example, the thickness thereof is increased), the reactivity is excessively increased around the middle burn-up stage. On the other hand, when the reflection ability is decreased, the increase in reactivity at the middle stage can be suppressed. However, the reactivity at the last stage is more decreased than that shown in FIG. 39.
The reactivity input by the reflector will be described in detail with reference to FIG. 40. FIG. 40 shows the change in reactivity caused by the movement of the reflector in the axial direction. The insertion depth of the reflector in FIG. 39 corresponds to distance between top of reflector and bottom of reactor core. As described above, in operation at the initial burn-up stage, the degree of insertion of the reflector is approximately 40% (insertion depth of 75 cm), and when the degree of insertion is increased from the state described above, the reactivity is approximately linearly increased. However, when the degree of insertion exceeds 60% (insertion depth of 120 cm), the increase in reactivity tends to be saturated. Hence, as shown in FIG. 39, the rate of increase in reactivity by the reflector is gradually decreased from the middle burn-up stage toward the last stage. The reasons for this tendency are that the reactivity value of the reflector is highest at the center of the reactor core at which the neutron flux is high, and that an effect obtained when the periphery of the reactor core is newly covered with the reflector becomes smaller than that obtained when the central portion of the core is newly covered with the reflector.
By the reasons described above, as long as the lifting speed of the reflector is maintained constant, the decrease in reactivity caused by the burn-up of core fuel cannot be totally compensated for by the movement of the reflector in the axial direction. Since the decrease in reactivity causes the decrease in thermal power, the thermal power with time cannot be maintained constant, and a small amount of thermal power can only be obtained around the last burn-up stage; hence, as a result, an uneconomic plant is disadvantageously to be constructed.
The tendency of the change in reactivity by the burn-up may be changed depending on design of the reactor core in some cases. However, unlike the case shown with reference to the above example, even when plutonium is used instead of the uranium concentrate, or even when the relationship between the length of the reflector and the length of the reactor core, which is determined in accordance with an operation period, is changed so that the length of the reflector may be larger or smaller than that of the core, it has been known that the tendency of the decrease in total reactivity around the last burn-up stage is not substantially changed.
One method for overcoming the decrease in reactivity described above is to control the lifting speed of the reflector with time. However, the control of the lifting speed may cause accidents in some cases due to excessive increase in reactivity resulting from malfunctions or failures of control devices. In order to avoid the accidents described above, a proposal has been disclosed in Japanese Patent No. 3131512 in which reactivity feedback is used which is generated by changing an inlet temperature of a coolant using the control of a flow rate of water supplied to a steam generator so as to maintain the power at a constant rate. In this Japanese Patent No. 3131512, the flow rate of water is controlled in accordance with thermal power of the steam generator, and the inlet temperature of a primary coolant is controlled through a secondary coolant, an intermediate heat exchanger, and the primary coolant. By this temperature feedback, it is intended to maintain the power of the reactor core at a constant rate.
However, the range of the thermal power which can be controlled by the control of the flow rate of water supplied to the steam generator is limited, and the control described above can be effectively performed only when the range of variation in thermal power is reduced to a small value by the reactivity control carried out only by the reflector. In addition, in the case in which a method for controlling a lifting speed of a reflector is carried out without performing the control described above, in view of safety, it is important that a time required for the control and a range of the lifting speed therefor be reduced as small as possible. Hence, as is the case described above, the range of variation in thermal power must be reduced to a small value by the reactivity control carried out only by the reflector.